What wut said...(apart from maybe going to windows 2k....don't touch that with a shitty bargepole

).....
Dick, what virus software are you using, and more importantly, have you updated it regularly?? (most AV progs should be updated at minimum once a week, as new virii are invented and get released into the wild)
Secondly, as wut said, have you defragmented your drives???
As I remember, in '98, it's under accessories, tools, defrag or something similar...but if you haven't done a defrag recently, try that first.
Look at your start menu and see what's there....although you did say you haven't loaded any new software recently so that sounds unlikely, but remove anything that you don't always use and start them from the start button rather than have them run in the background (I'm thinking of stuff like ICQ etc)
next, what do you have on your desktop????
If you've saved any piccies or progs on it, then those pics etc will use up your RAM all the time........anything on the desktop is held in RAM rather than on your hard drive......
When the amount of stuff you have on your desktop gets big enough, the PC will start using the hard drive as RAM and it takes much longer to get stuff off your HD than from RAM......
SO, look at your desktop....save any pics there onto your hard drive instead and the same with programmes, and make shortcuts to them on your desktop if you need to. (each shortcut will only use 1kb ram instead of whatever MB the prog or pic took p on the desktop in RAM)
If all this fails, the first thing I'd do is save all your emails, files, documents, settings etc etc onto CD or wherever you can (network? laptop? webspace?) and reload windows......even just doing that should clean up the registry and make things a lot faster, but if you reformat the drive as well, then reload windows, then unless the hard drive itself or your ram chips are buggered, you should restore the speed that you had when it was new.
(PS I reload windows at least twice a year as a matter of course, and it always makes me grin how much quicker it all is

)
PS......go out and buy yerself a new pc mate.......300 quid buys you an all singing all dancing jobbie that will be four times faster than your current one if you know where to look
